The general objective of our research is to study the biological foundations of human language. We investigate language and its formal architecture, as well as its representation in the brain by studying languages which have arisen outside of the mainstream of spoken languages: the visual gestural systems developed among deaf people. American Sign Language (ASL) displays complex linguistic structure, but unlike spoken languages, conveys much of its structure by manipulating spatial relations, thus exhibiting properties for which each of the hemispheres of hearing people shows a different predominant functioning. The study of deaf signers with left or right hemisphere damage offers a particularly revealing vantage point for understanding the organization of higher cognitive functions in the brain, and how modifiable that organization may be. We propose four major series of experimental studies, each focusing on a special property of the visual gestural modality as it bears on the investigation of brain organization for language: 1) The Role of the Left Hemisphere for ASL Grammar. We propose an in depth evaluation of deaf signers having either left or right brain damage. WE will investigate the relative contributions of each of the cerebral hemispheres to left or right brain damage. We will investigate the relative contributions of each of the cerebral hemispheres to language, with special reference to the linguistic functions and the spatial mechanisms that convey them. Specifically, we will examine the functioning of the two cerebral hemispheres for differential uses of space in ASL; from spatially organized syntax to the use of articulatory sign-space to represent external world-space. A variety of techniques will be brought to bear, including the investigation of the moment to moment on-line construction of linguistic and spatial mental representations. 2) The Role of the Right Hemisphere in Extra-Syntactic aspects of ASL. In sign language, facial signals serve two opposing functions: specifically linguistic as well as effective. Pitting form against function, we will investigate whether these two different functions are differentially lateralized in ASL. 3) Brain Organization for Spatial Cognition and Imagery. Since spatial relations and linguistic relations are so intimately intertwined in ASL, we will examine brain organization for nonlanguage for nonlanguage spatial functions, asking whether the two cerebral hemispheres in deaf signers show differential specialization. Paradigms will be developed that allow experimental investigators of attentional functions, and assess capacities for components of imagery, as imagery may play a unique role in sign language syntax and discourse. 4) Neural Mechanisms for Sign Language. We will investigate the dissociability apraxia from sign aphasia through experiments designed to separately evaluate impairments in linguistic, symbol and motor functions as well as to analyze the neural correlated for sign language, Summary. The essential issue and heart of this proposal is the nature of cerebral specialization for a visual- gestural language. By illuminating this issue we hope to shed light on theoretical questions concerning the determinants of brain organization for language general.